To obtain chips of uniform length in a chipping process, it is important to ensure smooth and uniform feeding of the pieces of timber to be cut between two consecutive chipper knives. The feeding of the timber can, at least to some degree, be controlled by a timber feeding device, but also by the timber-guiding surface formed between two chipper knives, which surface should follow an ideal cam curve for optimal guiding of the pieces of timber. The timber-guiding surface is formed on the one hand by a timber-guiding surface of the chipper knife forming a cutting edge and, on the other hand, by the outside of an outer clamping member, if any, but may also, in some types of machines, comprise a timber-guiding surface on the chipper disc between two consecutive chipper knives.
For chippers of the kind referred to above, in which the rotating tool has the shape of an essentially plane or conical disc and the respective chipper knives thus have a radial extension and are arranged with one end of the processing cutting edge closer to the axis of rotation of the disc than the opposite end, this means that the ideal cam curve will have a different shape or angle relative to the direction of rotation depending on the distance from the axis of rotation. The reason for this is that the distance along which the wood is advanced between two consecutive chipper knives should be the same though the distance between them along a radial line varies with varying radial distances from the axis of rotation. For example, in the case of a tool in the form of a plane disc, the ideal cam curve is rectilinear, as seen in cross-section, along one and the same radial distance from the axis of rotation, but the angle to a plane of rotation increases with decreasing radial distance from the axis of rotation. On the other hand, in the case of a conical chipper disc, the ideal cam curve is arched with a radius of curvature that decreases with decreasing radial distances from the axis of rotation.
The cutting edges of the chipper knives for such chippers are formed between two surfaces shaped by grinding or in any other way, a first surface or timber-guiding surface facing outwards towards the pieces of timber that are fed to the tool for cutting, while a second surface or chip-guiding surface faces inwards towards an opening formed in the tool and is adapted to guide the chips into the opening so that the chips can be discharged through the tool for direct use or intermediate storage. Consequently, the outwardly oriented timber-guiding surface of the cutting edge will form part of the cam curve that is adapted to guide the pieces of wood to the following chipper knife. Thus, the timber-guiding surface of the chipper knife should follow the ideal cam curve as closely as possible.
However, the chipper knives, in particular their cutting edges, are subjected to considerable wear, which means that it is also important that they should be made as inexpensive and simple as possible, so that they can be replaced and/or resharpened at a low cost. It is therefore quite common for the chipper knife to have a plane timber-guiding surface with a constant angle along the whole length of the chipper knife. As a rule, however, it is made as short as possible in order to interfere as little as possible with the feeding of the pieces of timber. A common type of chipper knife thus has an essentially flat shape with two large main surfaces, and the cutting edge is formed between one of the main surfaces and a bevelled surface between the main surfaces, the bevelled surface forming the timber-guiding surface, while the edge-forming main surface forms a chip-guiding surface.
The principles outlined above are known from U.S. Pat. No. 2,183,224, which thus discloses a chipper in which the edge-forming timber-guiding surface of the chipper knife has a shape that essentially corresponds to the ideal cam curve. In this case, the timber-guiding surface is formed by one of the main surfaces of the chipper knife, while the chip-guiding surface is formed by the bevelled surface between the two opposite main surfaces. However, this type of chipper and chipper knife has never been very commercially successful. Possibly because, on the one hand, the chipper knives are too expensive to manufacture and, on the other hand, the improvement in chip quality and the resulting increase in revenue have not been enough to compensate for the increased costs of the machine and the chipper knives.
SE 419,522 discloses a chipper in which a twisted timber-guiding surface is provided with a varying angle relative to the direction of rotation depending on the distance from the axis of rotation. In this case, this is achieved by using plane and straight inner knife holders, chipper knife and outer knife holders which are twisted by clamping them against a non-plane support. A drawback of a chipper of this kind is, however, that very large forces are required to deform the knife holder elements and the chipper knife, which means that mounting the chipper knives is very time-consuming. Moreover, when fixing the knife holder elements and the chipper knife in the twisted shape tension is built into the material, which makes them more susceptible to damage during operation.